The present invention relates to an improvement of a cleaning unit to be used in an electrophotographic copying machine or in an electrostatic recording apparatus, and more particularly, to an improvement of a magnetic brush type cleaning unit which removes toners staying on an image carrier with a magnetic brush.
An electrophotographic copying machine of a transfer type requires a cleaning unit that removes residual toners on an image carrier for the purpose of preparing for the succeeding development after a toner image has been transferred onto a transfer sheet, and a magnetic brush is used as one of the cleaning means in some copying machines.
However, there have been problems in a magnetic brush type cleaning means.
The first problem is that the magnetic brush type cleaning means is weak in force to sweep the surface of an image carrier compared with a conventional blade type cleaning means, and therefore, it is difficult for the magnetic brush type cleaning means to remove thoroughly the residual toners, though it has an advantageous point that it hardly hurts the surface of the image carrier.
With an aim to solve the problem mentioned above, Japanese Patent Examined Publication No. 59587/1989 (1-59587) discloses a magnetic brush type cleaning device wherein DC and AC bias voltage are impressed between an image carrier and a magnetic brush sleeve.
Compared with a conventional magnetic brush type cleaning unit, the magnetic brush type cleaning device mentioned above can exhibit higher cleaning effect because mechanical sweeping force as well as electric Coulomb force are used for cleaning.
However, when using toners having a small particle size, especially when using toners of spherical particles having a size of not more than 7 .mu.m, it has been difficult to remove thoroughly the residual toners on the surface of an image carrier even when the magnetic brush type cleaning device mentioned above is used. For the reason mentioned above, development of a cleaning device capable of coping fully with toners of smaller size particles which are considered to become popular more and more in the future.
The second problem of the magnetic brush type cleaning means is that the releasing of the cleaning means is complicated.
For example, in an electrophotographic copying machine having a process wherein an image carrier is rotated several times for superposing toners, when a cleaning member is kept to continue sweeping an image carrier constantly, a visible image is wiped off before it is transferred, which is a problem. In the past, therefore, there has been a switching means wherein a sleeve on which a magnetic brush is formed is made to be movable, and thereby the distance between the sleeve and an image carrier is changed so that an ear of the magnetic brush may be in pressure-contact with or be away from the image carrier.
When such switching means as stated above is used, however, the distance between the magnetic brush sleeve and the image carrier for cleaning is not stable, being changed each time the cleaning means is released, which can cause insufficient cleaning. In order to keep that distance at a certain accuracy constantly, in that case, a mechanism for releasing the cleaning means tends to be complicated, resulting in undesirable conditions in terms of maintenance, a size of an electrophotographic apparatus and price thereof.